The Pentagon’s effort to nail down $400 billion in cuts has been so tough that doubling that goal “would be extraordinarily difficult and very high risk,” the nominee to be the next top U.S. military leader told lawmakers.

Gen. Martin Dempsey, nominated to replace Adm. Michael Mullen as the next chairman of the Joint Chiefs of Staff, spoke to the Senate Armed Services Committee Tuesday. In the past year, Dempsey has skyrocketed through the ranks, moving from the commander of Army Training and Doctrine Command to Army chief of staff and now the nominee to lead the entire military.

Dempsey is slated to take over at a time when the White House and Congress are divided over a dizzying array of debt-reduction proposals, which hand down varying levels of cuts to the Pentagon. Those started out last year as an in-house call by Defense Secretary Robert Gates to reduce overhead costs by $178 billion so that money could be reinvested in higher priorities. By April, President Obama was calling for a $400 billion cut to defense over 12 years. Budget proposals under consideration by lawmakers would hand down nearly $900 billion in security spending reductions over 10 years. And one floated by Senate Majority Leader Harry Reid (D-Nev.) is trumpeting $1 trillion in savings by drawing down forces in Iraq and Afghanistan.

Sen. John McCain (R-Ariz.) says that although he is often a critic of wasteful Pentagon spending, any reductions to the military budget should be strategy driven. And he asked Dempsey for his thoughts on an $800 billion cut to defense.

“I haven’t been asked to look at that number. But I have looked and we are looking at $400 billion,” Dempsey said. “Based on the difficulty of achieving the $400 billion cut, I believe achieving $800 billion would be extraordinarily difficult and very high risk.”

As Dempsey pointed out, his thoughts on what to keep and what to cut are evolving to meet budget-cutting demands.

Sen. Jeff Sessions (Ala.), who is also the top Republican on the Senate Budget Committee, asked Dempsey why he did not support bringing one of three brigades home from Europe. After explaining the importance of international relationships, particularly when budgets are tight, Dempsey said his previous comments were made when the Pentagon was looking at how to pull $178 billion out of the budget — not $400 billion. In the new context, Dempsey said, “Everything is back on the table.”

Based on Dempsey’s comments, Sen. Joseph Lieberman (I-Conn.) told Aviation Week that a debt-reduction deal should stick to about $400 billion in cuts to the Pentagon’s overall budget, along with a reduction in war costs. But he acknowledged even that will not be a smooth ride, pointing out that Dempsey said the military has been using war funding for things that should really be in its base budgets. “So it’s going to be hard,” Lieberman said.